Compound of 室(muro, “cellar; room”) +‎ 積み(tsumi, “piling up, heaping up, stacking up”, the 連用形(ren'yōkei, “continuative or stem form”) of verb 積むtsumu, “to pile, heap, or stack up”),[1] possibly in reference to the way that the oldest such buildings may have started out as simple pithouses.